The invention relates to a differential delay measurement circuit, and more particularly, to a delay measurement circuit suitable for use in a bearing deviation indicator for sensing the delay between portions of a sonar transducer array to provide a measure of the angle of orientation of an incident sound wave relative to the transducer array.
Bearing deviation indicators have been utilized in sonar applications for measuring the angle of orientation of a beam of sound relative to an array of transducers. Bearing deviation indicators typically comprise a differential delay circuit having two input channels for receiving signals from each half of an array of sonar transducers. The delay sensed between signals of the two channels is readily converted to the direction of the incident sonar beam. Such differential delay circuits are utilized in other applications besides sonar, and may be utilized, for example, for measuring the differential delay between two signals in a radar system. However, to facilitate a description of the invention, the differential delay circuit will be described with reference to its use in a bearing deviation indicator for sonar application.
A delay measurement circuit used in bearing deviation indicators is responsive to the leading and/or trailing edges of signal pulses in each of the two channels. Typically, the pulse type signals in each of the channels are produced by limiters or clipping amplifiers coupled between the sonar transducers and the delay measuring circuit.
A problem arises in the use of a bearing deviation indicator with sonar receivers having bandwidths in excess of approximately two octaves, and wherein the ratio of signal power to noise power (SNR) is less than approximately 5 dB. The problem is manifested by jitter which may be present in the leading and trailing edges of the signal pulses. The jitter is present when the centroid of the power spectrum of the signal is at a lower frequency than the centroid of the noise spectrum. Exemplary situations are that of a monotone sonar signal at a frequency in the lower portion of the receiver passband or a sonar signal having the major portion of its power falling within the spectral region at the lower portion of the receiver passband while the noise power uniformly fills the receiver passband.
In the foregoing situations, even a small difference in the amounts of receiver noise between the two channels of the bearing deviation indicator produces a bias error in the delay measurement. An apparent angular offset of the incident sonar beam is thereby registered even when the beam axis coincides with the array axis. Due to the jitter, the average number of axis crossings of the signal in one channel differs from that of the signal in the other channel, the difference being dependent on the relative magnitudes of the SNR in one channel as compared to the SNR in the other channel.